The railroad-highway crossing, at a common grade, presents a potentially dangerous situation. Highway crossing warning systems have heretofore been developed to provide a warning to highway users of the approach of a train, with the desired goal of insuring that the crossing is clear at the time the railroad vehicle passes thereover. The problem of providing a safe and effective warning system is complicated by a number of variable factors.
AAR recommendations suggest that a minimum 20 second warning time be given of the approach of a train. Because the highway crossing warning system has no control over the speed of the approaching railroad vehicle, it must accommodate its operation to the motion of the railroad vehicle which can slow down or speed up as it approaches the crossing, indeed, the vehicle can even stop and start up again, such motion can be toward or away from the crossing. Furthermore, after the railroad vehicle has passed the crossing, the railroad vehicle may slow down, speed up, stop and then even reverse its motion and re-cross the crossing. The ideal highway crossing system should provide a minimum warning time regardless of these variations.
Further complicating the design of these systems is the variability which is inherently present under normal operating conditions. For example, one typical method of detecting the presence of a vehicle is the track circuit. The track circuit employs a source of electrical energy (DC, AC or frequency shift) which is applied to the track rails at one point and an electrical energy detector, such as a relay or other receiver, which responds to the energy impressed on the rails by the transmitter. The presence of a conventional railroad vehicle, with the steel wheel shunt it provides, alters the energy detected at the receiver, and this alteration is usually employed to signal the presence of a train. The track circuit is, however, subjected to variables other than the presence or absence of the train. For example, the track circuit is shunted through the ballast on which the rails are supported. This effective shunt is variable depending, for example, on moisture conditions. Furthermore, the conductivity of the track rails themselves may change their conductivity characteristics due to a variety of factors. One such factor, for example, is the presence or absence of rust in local spots on the rail.
Another type of arrangement which has recently become popular in highway crossing warning systems is the train motion detector. Whereas the track circuit employed the gross change in track circuit conditions caused by a train entering or leaving the track circuit to detect the presence or absence of the train; the motion detector, instead, relies upon the voltage variations at a receiver, as a train approaches or leaves the point at which the receiver is connected to the track rails, to detect train approach or departure. That is, train velocity is implied from the rate of change of voltage detected by the receiver. The variable factors affecting the track circuit also affect this type of operation.
Many of the older highway crossing systems employed insulated track sections. With the popularity of the welded rail, and the associated desire of the railroads to eliminate insulated joints, however, there is a desire to use non-insulated track circuits in the highway crossing warning system. As those skilled in the art will appreciate, the lack of insulated joints provides further variable factors inasmuch as now the changes in weather conditions can not only affect the nominal operating points, but can also affect the "range" within which vehicles can be detected.
Since the motion detector relies on amplitude information for motion detection, failures in the apparatus which have, or may have, the effect of increasing the modulation or amplitude level of the received signal are particularly dangerous. This is true for such failures could "mask" a reduction in the same quantity caused by approaching motion and thus prevent motion detection, when such detection should occur. It is therefore one object of the invention to provide a motion detector with a transmitter so arranged that failures in the transmitter circuitry will not result in increasing the modulation level or amplitude level of the transmitted signal.
Likewise, if a motion detector transmitter includes linear amplification circuitry, for example, to eliminate square wave harmonics to produce a sinusoidal signal, failure modes of that linear amplification circuitry could result in increasing the amplitude levels of the modulation signal or carrier. It is therefore another object of the present invention to provide a motion detector transmitter which eliminates the necessity for a linear amplifier.
Other failure modes in a transmitter include changes in oscillator frequency, failure of switching components such as dividers or switching transistors, and spurious high frequency oscillations. It is another object of the present invention to provide a motion detector transmitter which has circuitry arranged to prevent oscillator drift, failure of dividers or switching transistors, or spurious high frequency oscillations from producing increases in signal levels.
Prior art motion detector transmitters produce a modulated signal, i.e., a relatively high frequency carrier modulated by a lower frequency signal. If the carrier and modulating signal are separately generated, changes in relative phases of the modulating and carrier signals can result in variations in the transmitted signal of a relatively low frequency nature. Variations can produce the same effect on the motion detector as that of a train slowly oscillating back and forth in position. Hence, these variations may be considered as system noise which tends to mask the real signal and limits usable receiver sensitivity. It is therefore another object of the invention to provide a motion detector transmitter in which the modulating signal and carrier are synchronously generated, i.e., they are phase locked, thus eliminating low frequency variations in the transmitted signal as a result of relative phase changes between modulating and carrier signals and improving system signal to noise ratio.
As pointed out above, the motion detector responds to the modulating signal level. It is another object of the present invention to provide a motion detector in which the motion detector has a filter to prevent carrier energy from reaching the motion detector.
As pointed out above, some prior art highway crossing systems include wrap-around circuits in addition to a motion detector. Furthermore, it is conventional in highway crossing systems to include a detector for a so-called "island" circuit which is immediately adjacent the highway crossing on either side thereof, to insure that the crossing warning is energized whenever a vehicle is within the "island" region. The "island" receiver also serves to "reset" the logic of the highway crossing system as the train crosses over the crossing. Typical prior art systems included both a transmitter for the motion detector and a different separate transmitter for the island receiver. It is an object of the invention to provide a highway crossing warning system in which the motion detector transmitter energizes not only the motion detector, but also the island track circuit receiver eliminating the necessity for a separate island transmitter.